In recent years calcium carbonate has found a wide array of uses across many fields. For example, calcium carbonate is one of the most widely used minerals in the paper, plastic, paint and coating industries both as a filler and, due to its white color, as a coating pigment. In the paper industry calcium carbonate is valued for its high brightness, opacity and gloss and is commonly used as a filler to make bright opaque paper. In addition, calcium carbonate is frequently used as an extender in paints and is also used as a filler in adhesives and sealants. High grade calcium carbonate has also found uses in formulations of pharmaceuticals.
Calcium carbonate is known to exist as three types of crystal polymorphs: calcite, aragonite and vaterite. Calcite, the most common crystal polymorph, is considered to be the most stable crystal form of calcium carbonate. Less common is aragonite, which has a discrete or clustered needle orthorhombic crystal structure. Vaterite is the rarest calcium carbonate polymorph and is generally unstable.
Among these three forms, aragonite is particularly useful in a number of applications, such as an inorganic filler or paper coating, and due to its needle-shaped crystal structure it offers enhanced paper bulk and opacity. As a result, the demand for calcium carbonate with a significant aragonite crystal content has increased substantially.
Generally, one way to produce calcium carbonate commercially is by calcining crude calcium carbonate to obtain quicklime. Water is then added to yield an aqueous suspension of calcium hydroxide (“milk of lime”), and carbon dioxide is reintroduced into this slurry to precipitate the calcium carbonate. The product of this process is known as precipitated calcium carbonate (“PCC”). The resulting aqueous suspension, or slurry, of calcium carbonate may be used as is or further processed (i.e. dewatered, ground etc.) to form a dry product. The precipitation reaction is capable of producing each of the three polymorphs (calcite, aragonite and vaterite) depending on the exact reaction conditions used.
Prior art processes for producing a PCC product with an increased aragonitic crystal content have generally taken a number of approaches. In a first approach, processes have been developed which use additives, such as strontium salts, to promote the production of the aragonitic crystal during carbonation resulting in the formation of PCC. For example, GB 2145074A discloses a process for producing a precipitated aragonite calcium carbonate product by adding strontium chloride (SrCl2) during carbonation to act as an agent in the formation of aragonite crystals. The use of strontium compounds as additives to the carbonation process for preparing PCC is also disclosed in the PCC production processes of the following Japanese Patent Publications: JP63260815, JP59223225, JP5116936 and JP1018911.
In a second approach, processes have utilized aragonite seed crystals to increase aragonite crystal formation. For instance, GB 941900A teaches a process for producing aragonitic calcium carbonate by introducing a sodium carbonate solution during carbonation. The reference suggests that the addition of aragonite seed crystals, to the disclosed process, may accelerate the production of aragonite crystals.
The production processes discussed above have been shown to increase marginally the total aragonitic crystal content of the PCC product. However, these processes have generally been unable to produce PCC having a total aragonitic content of greater than 60%.
EP 0 768 344 A2 describes a method for the preparation of a precipitated calcium carbonate for use as a pigment in paper coating companions. The method comprises the steps of (a) carbonating an aqueous lime-containing medium to produce an aqueous suspension of a precipitated calcium carbonate predominantly in a selected crystal form, (b) at least partially dewatering the PCC-containing suspension, and (c) subjecting the PCC-containing suspension to comminution by high shear attrition grinding with an attrition grinding medium.
Finally, the Applicant draws the attention to her own patent application WO 2011/121065 that also discloses the use of aragonite seed crystals to increase aragonite crystal formation. After the formation of the aragonite seed crystals, the aqueous suspension of seeds is subjected to a post-treatment consisting of a dewatering and grinding step before the seeds are used for the formation of the aragonitc PCC end product. However, the use of a grinding step is high in energy consumption and hence makes the production of the aragonitic PCC product expensive.
Accordingly, there still exists a need for a low cost process for producing precipitated PCC containing increased amounts of the aragonitic polymorph.